1. Field of the Invention
The present invention relates to gage control apparatus and methods for tandem rolling mills and in particular to an automatic gage control apparatus and methods based on the low of constancy of mass flow.
2. Description of the Prior Art
As one of gage control apparatus for the rolling mills, there has been hitherto well known a gage control system of rolling load feedback type, which is disclosed in U.S. Pat. No. 2,680,978 to Raymond Bernard Sims and widely employed in many practical applications. According to the principle of such type gage control system, a rolling load or pressure P and a screw-down position or a roll gap S are detected and the thickness of a plate material to be rolled at the output side of a mill stand (output thickness) is estimated in accordance with the following formula (Hooke's law): EQU h = S + P/M (1).
where
H: ESTIMATED OUTPUT THICKNESS,
M: rigidity coefficient of mill stand. When the estimated thickness is deviated from a desired thickness to be accomplished, the screw-down position or the roll gap S is adjusted so that the deviation becomes zero. This method is simple in control. However, it is known that an offset in gage will disadvantageously occur due to errors in the detected null or zero point of the screw-down position or the like factors. With an attempt to cancel out such an offset, it is common to dispose a gage meter at the output side of each mill stand and correct the screw-down position as a function of the offset quantity by feeding back the detected value from the gage meter. Besides, in the case of the control system of the rolling load or pressure feedback type described above, a detection of the rolling pressure P is necessary and thus involves indispensably such difficulties as described below.
Assuming that there exists a roll eccentricity, the roll gap, i.e. the gap between the rolls will be varied as the rolls are rotated even when the screwdown position which is set with reference to the axes of the rolls is constant. Such situation will prevail also during the rolling operation and the variation in the roll gap will appear as a variation in the rolling load or pressure as measured usually by a load cell. It will be known that the rolling load will increase, when the actual roll gap is reduced as the rolls are rotated. On the other hand, when the roll gap is increased during the rotation of the rolls, the rolling load is decreased. Since measurement of the roll gap S in the equation (1) during the rolling operation will encounter with a great difficulty in practice, the following method is usually adopted. Namely, a position at which the upper and lower rolls are snugly fitted without any material squeezed therebetween is taken as the zero point of the screw-down position and the roll gap S is estimated on the basis of the difference between the zero point and the set screw-down position. Accordingly, the roll gap S of the equation (1) will constitute a constant at the step at which the screw-down position has been set before the pass of the late material to be rolled. The rolling load is increased when the actual roll gap is being decreased due to the eccentricity of the rolls, and the estimated thickness h as derived from the formula (1) will be increased. The control system would operate to adjust the screw-down position and rotation speeds of the rolls so that the deviation of the estimated thickness from a desired thickness to be attained may become zero. Therefore, notwithstanding the fact that the thickness of the rolled material at the output side of the mill stand is really decreased, the control would be effected so as to more reduce the thickness of the rolled material by enforcively lowering the screw-down position in response to the increase in the rolling load. On the contrary, when the roll gap is increasing due to the eccentricity of the rolls, the control is carried out in such a manner that the thickness of the rolled material will be undesirably increased.
As will be appreciated from the above description, the rolling load feedback type control system responds to the influence of the roll eccentricity in the reversed sense. For the similar reason, the above control system responds to the variation in the radius of the rolls such as caused by thermal expansion of the roll diameter in the reversed sense, i.e. the control system operates to exaggerate the adverse influence of the roll eccentricity rather than compensate it.
To do away with the above problems, it is conceivable to dispose the thickness measuring devices i.e. gage meters at both the input and the output sides of each of the mill stands and to effect the control operation with the aid of the detected values from these devices. However, such an arrangement of the rolling mill will necessarily involve high expensiveness in addition to difficulty that the arrangement can not be applied to the existing plants since no extra spaces are available for installing the gage meters.
Under these circumstances, a gage control method based on the law of constancy of mass flow has been developed. However, the hitherto known control systems utilizing such principle are not always satisfactory in respect of the attainable accuracy because of insufficient analysis of the actual rolling phenomenon.